Yes, fm stereo is strong, but it lights only when aux, phono or tape head are selected: normal or abnormal?
Sherwood S8000 IV Rehab
- Thread starter Dave451
- Start date
basplin
Active Member
I understand now - that is very abnormal. I'm not really sure what I would do here, except search for bad resistors/caps in the area, which it looks like you've been doing. Perhaps realigning the FM section would be helpful too. That's an area that I really don't know much about...
Thanks basplin, probably faulty components and drifted alignment...
Back in town, guys and resumed work on the S8000 IV. Interesting discussion on lights. I bought replacements for the incandescent lamps--I figure at least one is gone after I power it up. The S8000 IV has the neon stereo indicator as well, but of course I haven't been able to check it yet.
Anyway, my parts order came in so I've been working on replacing electrolytic caps. Found that the actual cans (all originals) were not always the same as the schematic (flash back to the power transformer wiring!), as follows:
1. Replaced C208 50+50 uF 500 VDC with JJ 50+50 500VDC unit. Used a 1 3/8 inch clamp to mount the JJ, as the original was mounted with tabs directly to the chassis and the JJ has no tabs.
2. Replaced C206 (120 uF/275 VDC + 40 uF/275 VDC) with CE brand 50uf x 4 @ 350 VDC multi-section. I paralleled three of the 50 uF sections for 150 uF to replace the 120 uF section and used the remaining 50 uF section to replace the 40 uF section. Mounted this directly to chassis.
3. Replaced C207 (30 uF@ 425VDC + 30 uF @ 350 VDC + 20 uF @ 250 VDC + 60 uF @ 200 VDC) with a CE unit 3 X 30 uF + 10 uF all at 475 VDC and paralleled a 47 uF 250 VDC axial cap with the 10 uF section for 57 uF to replace the 60 uF section. NOTE: the schematic called for 40 + 30 + 20 + 60 uF, but that was not what was actually mounted in the unit.
4. I replaced C 205 and C240 (both 120 uF 275 VDC) in the voltage doubler circuit with two F&T cans: 100 uF x 2 @500 VDC and 32 uF at 500 VDC and some below chassis axials to get near the proper amounts. I outsmarted myself on this one, figuring that I'd parallel a 100 uF and 32 uF section from each cap to have two 130 uF equivalents to replace the 120 uF originals. Unfortunately for my plan, each can (of course) has one negative terminal and this prevented me from chaining the the two paralleled sections together as you must do for the voltage doubler. SO, I used one section of the 100 uF F&T can paralleled with a 22 uF 500 VDC axial for C205. I then paralleled the two 32 uF sections in the other F&T can and added, also in parallel, two 40 uF axials for a total of 140 uF for replacing C240. This wasn't as elegant as I originally planned, but should enable me to power up and test and may well be OK permanently. The axials were small enough to mount them under the chassis. Things are tight, but should be OK. Had to work carefully.
I installed a CL-80 thermistor in rush current limiter in the power transformer primaries (there are two in parallel for the 234/117 VAC transformer--see above posts). As above, it is also tight around it but I was able to keep enough free space around it, I think. These are, after all, resistors and they generate a fair amount of heat during operation, so you have to keep them separated from other components.
I also replaced C54, the 50 uF 3 VDC cathode bias cap for the 6EU7 pre-amp section with 50 uF 16 VDC.
I'll post some pictures tomorrow. Going after the bias supply electrolytics next. I think I'll leave the silicon diodes for the HV and the bias supplies alone for now (no selenium in this unit) and do some careful no-tube testing with the variac to see if things are OK before proceeding with more work.
Dave
Anyway, my parts order came in so I've been working on replacing electrolytic caps. Found that the actual cans (all originals) were not always the same as the schematic (flash back to the power transformer wiring!), as follows:
1. Replaced C208 50+50 uF 500 VDC with JJ 50+50 500VDC unit. Used a 1 3/8 inch clamp to mount the JJ, as the original was mounted with tabs directly to the chassis and the JJ has no tabs.
2. Replaced C206 (120 uF/275 VDC + 40 uF/275 VDC) with CE brand 50uf x 4 @ 350 VDC multi-section. I paralleled three of the 50 uF sections for 150 uF to replace the 120 uF section and used the remaining 50 uF section to replace the 40 uF section. Mounted this directly to chassis.
3. Replaced C207 (30 uF@ 425VDC + 30 uF @ 350 VDC + 20 uF @ 250 VDC + 60 uF @ 200 VDC) with a CE unit 3 X 30 uF + 10 uF all at 475 VDC and paralleled a 47 uF 250 VDC axial cap with the 10 uF section for 57 uF to replace the 60 uF section. NOTE: the schematic called for 40 + 30 + 20 + 60 uF, but that was not what was actually mounted in the unit.
4. I replaced C 205 and C240 (both 120 uF 275 VDC) in the voltage doubler circuit with two F&T cans: 100 uF x 2 @500 VDC and 32 uF at 500 VDC and some below chassis axials to get near the proper amounts. I outsmarted myself on this one, figuring that I'd parallel a 100 uF and 32 uF section from each cap to have two 130 uF equivalents to replace the 120 uF originals. Unfortunately for my plan, each can (of course) has one negative terminal and this prevented me from chaining the the two paralleled sections together as you must do for the voltage doubler. SO, I used one section of the 100 uF F&T can paralleled with a 22 uF 500 VDC axial for C205. I then paralleled the two 32 uF sections in the other F&T can and added, also in parallel, two 40 uF axials for a total of 140 uF for replacing C240. This wasn't as elegant as I originally planned, but should enable me to power up and test and may well be OK permanently. The axials were small enough to mount them under the chassis. Things are tight, but should be OK. Had to work carefully.
I installed a CL-80 thermistor in rush current limiter in the power transformer primaries (there are two in parallel for the 234/117 VAC transformer--see above posts). As above, it is also tight around it but I was able to keep enough free space around it, I think. These are, after all, resistors and they generate a fair amount of heat during operation, so you have to keep them separated from other components.
I also replaced C54, the 50 uF 3 VDC cathode bias cap for the 6EU7 pre-amp section with 50 uF 16 VDC.
I'll post some pictures tomorrow. Going after the bias supply electrolytics next. I think I'll leave the silicon diodes for the HV and the bias supplies alone for now (no selenium in this unit) and do some careful no-tube testing with the variac to see if things are OK before proceeding with more work.
Dave
Good work and surely neater than the work I've done on the 7700.
I've left the silicon diodes too and some unattainable caps...
Thanks, Patrice. By the way, every one of the cans checked bad for electrical leakage current at max working voltage and two where physically leaking around the connectors with bulging vents, so they all had to go! This is the first amplifier or receiver I have worked on that had every one of the can caps check bad. The new cans and tubes will comprise most of the rehab budget for this (figuring I do get it working!).
Dave
Dave
same deal with my S-7000. The doubler caps were leaking goo, the others were just NFG. My S-5500II actually seems to be OK in that department. Its running original cans at the moment and it works fine. They were only made 3 years apart and neither gives indication of particularly bad storage conditions, so no idea why the difference.
Good day today with the S8000 after a couple of set backs! Completed the following:
1. Replaced bias supply C209 filter caps. One was dead and the one left was a replacement 2X the value. I put in two 25 uF 50 V caps.
2. Unfortunately, I apparently broke one of the very tiny signal diodes in the FM ratio detector attached to transformer T7. These diodes are incredibly small and fragile (see photo) and I must have broken X3 when I was working on C207 in the vicinity. I have a number of replacement types of these germanium signal diodes, and decided to replace both with a matched pair of 1N34A replacements (NTE DO-7) and crossed my fingers.
After that, I decided to power up without tubes and check to see if I was getting B+ and bias voltage. Brought everything up very slowly on the Variac and dim bulb tester to watch for shorts and keep the voltages pretty low without load and everything checked out and no smoke! So, I put the tubes in; the originals plus a NOS matched quad of Brimar EL506's I bought just for the Sherwood. I set the bias pots for each side to maximum negative voltage (max CW).
The Brimar's have the Magnoval size pins (like the Electro Harmonix 7868's), so I used my solder pick to gently enlarge the pins in the sockets before inserting the Brimar's. They fit very well--tight, but not too tight and I avoided having to push and shove on them to get them in.
After hooking up the bench speakers and DMM's to measure line voltage I powered up and, dang, the thing works!
I did a quick check of key voltages and most look OK (measured shown with spec in parentheses):
Line voltage was 120 VAC before the CL-80 and 118 VAC after it.
B+ supply: 454 (445) VDC
Power tube plates: 451 (438)
Power tube screens: 435 (425)
Driver supply: 425 (420)
Tone amp: 355 (345)
Pre-amp: 213 (200)
B+ point 6: 191 (170)
B+ point 7: 155 (115).
Bias supply: -23.1
Power tube control grids: -22.7
I don't have cathode resistors in place yet, but the schematic says 23V on the control grids (I presume this should be -23V), so we're right in the vicinity with the bias pots all the way negative. The Brimar's must be pretty hot tubes. The point 7 voltage is high, so I'll need to check that out. The 82K resistor in the supply string up to it measured fine, but will do some more checking.
That's the data, but I have to say the sound was pretty darn good, too. FM mono came through clear (the ratio detector diodes must be working!) with the local college station booming through with just a clip lead on the antenna (transmitter is only about 10 or 15 miles away). Stereo indicator light and the signal meter both worked. Stereo came through at considerably more volume in the same volume control position and sounded great and like stereo, but it would cut out after a little while sometime, but returning the mode switch to mono FM brought back the signal every time. I know there are six weak tubes in the original set, including both front end tubes, the IF tubes, and all three multiplexer tubes. I'll put the new ones before judging radio performance and doing any alignment work, of course.
Played my iPhone in through auxiliary input and sounded very good into my cheap bench speakers. In fact, every input was dead quiet even at high volume levels, including the phono and tape head inputs. I didn't hear any hum or tube rush with these inputs with no input in place, but I'll have to signal check these inputs to be sure the pre-amp is actually working. I haven't heard a pre-amp yet that didn't have a little hiss/rush in phono or tape head. (come to think of it, the phono level control may have been all the way down--will have to check; not used to having one of these).
No indications of distress (red plating or otherwise) with the Brimar's. I notice the characteristic blue glow inside the plates around the screens which I always see on 7591 and 7868 type tubes and often on EL84 types, so I don't assume any trouble there (anyone disagree? see photo). No crackling or popping or hum whatsoever so far.
Other observations:
-All the panel, meter and mode indicator lights work
-The neon stereo pilot indicator works
-Pots and switches need cleaned. Things started working better when I worked the mode switch and pots, but a good cleaning of the switches, pots, and tube pins will be done
-All caps and transformers running cool after 30 minutes.
Pictures below, including the bench voltage test set up, new can caps top and bottom views, the tiny FM ratio detector diode (it's partner broke clean in half), the 1N34A replacements installed, and a dark shot of the Brimar's with glowing screens.
Well, pretty happy with things so far, but a lot more work to do.
Dave
1. Replaced bias supply C209 filter caps. One was dead and the one left was a replacement 2X the value. I put in two 25 uF 50 V caps.
2. Unfortunately, I apparently broke one of the very tiny signal diodes in the FM ratio detector attached to transformer T7. These diodes are incredibly small and fragile (see photo) and I must have broken X3 when I was working on C207 in the vicinity. I have a number of replacement types of these germanium signal diodes, and decided to replace both with a matched pair of 1N34A replacements (NTE DO-7) and crossed my fingers.
After that, I decided to power up without tubes and check to see if I was getting B+ and bias voltage. Brought everything up very slowly on the Variac and dim bulb tester to watch for shorts and keep the voltages pretty low without load and everything checked out and no smoke! So, I put the tubes in; the originals plus a NOS matched quad of Brimar EL506's I bought just for the Sherwood. I set the bias pots for each side to maximum negative voltage (max CW).
The Brimar's have the Magnoval size pins (like the Electro Harmonix 7868's), so I used my solder pick to gently enlarge the pins in the sockets before inserting the Brimar's. They fit very well--tight, but not too tight and I avoided having to push and shove on them to get them in.
After hooking up the bench speakers and DMM's to measure line voltage I powered up and, dang, the thing works!
I did a quick check of key voltages and most look OK (measured shown with spec in parentheses):
Line voltage was 120 VAC before the CL-80 and 118 VAC after it.
B+ supply: 454 (445) VDC
Power tube plates: 451 (438)
Power tube screens: 435 (425)
Driver supply: 425 (420)
Tone amp: 355 (345)
Pre-amp: 213 (200)
B+ point 6: 191 (170)
B+ point 7: 155 (115).
Bias supply: -23.1
Power tube control grids: -22.7
I don't have cathode resistors in place yet, but the schematic says 23V on the control grids (I presume this should be -23V), so we're right in the vicinity with the bias pots all the way negative. The Brimar's must be pretty hot tubes. The point 7 voltage is high, so I'll need to check that out. The 82K resistor in the supply string up to it measured fine, but will do some more checking.
That's the data, but I have to say the sound was pretty darn good, too. FM mono came through clear (the ratio detector diodes must be working!) with the local college station booming through with just a clip lead on the antenna (transmitter is only about 10 or 15 miles away). Stereo indicator light and the signal meter both worked. Stereo came through at considerably more volume in the same volume control position and sounded great and like stereo, but it would cut out after a little while sometime, but returning the mode switch to mono FM brought back the signal every time. I know there are six weak tubes in the original set, including both front end tubes, the IF tubes, and all three multiplexer tubes. I'll put the new ones before judging radio performance and doing any alignment work, of course.
Played my iPhone in through auxiliary input and sounded very good into my cheap bench speakers. In fact, every input was dead quiet even at high volume levels, including the phono and tape head inputs. I didn't hear any hum or tube rush with these inputs with no input in place, but I'll have to signal check these inputs to be sure the pre-amp is actually working. I haven't heard a pre-amp yet that didn't have a little hiss/rush in phono or tape head. (come to think of it, the phono level control may have been all the way down--will have to check; not used to having one of these).
No indications of distress (red plating or otherwise) with the Brimar's. I notice the characteristic blue glow inside the plates around the screens which I always see on 7591 and 7868 type tubes and often on EL84 types, so I don't assume any trouble there (anyone disagree? see photo). No crackling or popping or hum whatsoever so far.
Other observations:
-All the panel, meter and mode indicator lights work
-The neon stereo pilot indicator works
-Pots and switches need cleaned. Things started working better when I worked the mode switch and pots, but a good cleaning of the switches, pots, and tube pins will be done
-All caps and transformers running cool after 30 minutes.
Pictures below, including the bench voltage test set up, new can caps top and bottom views, the tiny FM ratio detector diode (it's partner broke clean in half), the 1N34A replacements installed, and a dark shot of the Brimar's with glowing screens.
Well, pretty happy with things so far, but a lot more work to do.
Dave
Attachments
Looks good. I was really anxious to hear those Brimar tubes, but one of the ones I bought was a dud and now they aren't listed. 3 doesn't do a lot of good unfortunately. They seem well made though, and when I tried them they did seem to bias up close to what an old stock one would. The Russian ones need more negative voltage.
Thanks, guys. Today's update:
1. Changed the bad tubes with NOS from Jim McShane. This improved operation of the FM receiver greatly with stable stereo and proper functioning of the deviation meter and the stereo (neon) indicator.
2. Did a power up and long listening period (2 hrs +) and am quite impressed with what I'm seeing and hearing.
Observations:
-Audio clarity in FM and AUX modes is excellent. No distortion in the stereo FM signal. Getting good separation--listening to classical music on the local college station and I can hear the separate individual vinyl pops on each channel. You have got to love a station that still broadcasts a Hi-Fi signal and actually plays vinyl on the air!
-I turned up the Phono Gain control and, yep, the pre-amp is working fine. There is a very slight tube rush at max gain and volume, but a good bit less than I'm used to hearing
-Right now, there is NO hum in anything! There are no covers installed at all and no tube shields in place and the 8000 has no hum discernible, at least, in my bench speakers. Gratifying, and we'll see how this holds up when I sound test it with the Klipsch speakers.
-The headphone audio is outstanding on FM and AUX sources. Classical music can be a real challenge because of the dynamic range and nature of the instruments (individual piano solo's will reveal flaws in audio reproduction). Bass is good a smooth in the phones on non-classical material through AUX.
-The Bimar EL506's are doing the job--these tubes are sounding really good and no untoward behavior so far (crackles, flashes, or glowing)--just good audio.
-Temperatures on the caps, transformers, and tubes all seem good after a couple hours of play.
So, the plan going forward (not necessarily in order):
1. Clean all switches, pots, and jacks. I have already done the RCA input jacks and the speaker terminals.
2. Replace couplers--at least the main couplers to the output tubes and maybe others. I have them all in hand, but the sound is pretty awesome right now, so I may choose to wait on the others until I listen through the main speakers (ANYONE HAVE EXPERIENCE WITH THE LIGHT BLUE COUPLER? THIS ONE HAS A BUNCH OF THEM BUT I HAVEN'T READ THEY ARE PARTICULARLY PRONE TO FAILURE).
3. Install 10 ohm cathode resistors and 100 ohm screen protection resistors. The bias level can be adjusted for each side, but not the balance between tubes. I will probably not mess with the bias circuit, but we'll see.
4. Re-install the dial cord and calibrate the dial.
5. Complete physical rehab, including cleaning up the front panel, maybe painting the tops of the transformers. I've cleaned the chassis and probably will leave it as it is now. It's clean and I did manage to get light rust off the worst parts (using a light and fast application of Naval Jelly followed by a fresh water rinse and polishing with Mother's polish. It looks good, but threatens the silk screened writing so I may just stop there. Clean up (or maybe paint) the tube shields.
6. Clean the wipers on the 3-gang tuning cap.
7. Re-install and re-solder the output tube shield (polished it up pretty nicely).
8. Fabricate at least a bottom cover from aluminum, probably, and install feet.
9. Further test the low-level inputs for response and hum (phono and tape head)
9. Depending on how the receiver acts when I hook it up to a real antenna, decide whether an alignment is needed. With 6 new tubes in the tuner section and new FM ratio detector diodes, this may be desirable. I have the tools to do it properly (including the MPX detector), but won't mess with it if the sensitivity, selectivity, and dial calibration seem OK. I'm thinking it will probably be necessary, though, at least for the receiver front end and IF strip. The multiplex decoder seems to be operating well and usually, if there is a problem with stereo, aligning the front end usually resolves it. We'll see.
10. Power and distortion test the amplifier.
I would appreciate any suggestions others may have about other work to do at this point, particularly from those who are familiar with Sherwood receivers or anyone with feedback or suggestions. I'm very pleased with what I'm hearing from this unit so far and look forward to getting it rehabbed and into the listening room for some longer-term listening and comparison with the other receivers I have.
Dave
1. Changed the bad tubes with NOS from Jim McShane. This improved operation of the FM receiver greatly with stable stereo and proper functioning of the deviation meter and the stereo (neon) indicator.
2. Did a power up and long listening period (2 hrs +) and am quite impressed with what I'm seeing and hearing.
Observations:
-Audio clarity in FM and AUX modes is excellent. No distortion in the stereo FM signal. Getting good separation--listening to classical music on the local college station and I can hear the separate individual vinyl pops on each channel. You have got to love a station that still broadcasts a Hi-Fi signal and actually plays vinyl on the air!
-I turned up the Phono Gain control and, yep, the pre-amp is working fine. There is a very slight tube rush at max gain and volume, but a good bit less than I'm used to hearing
-Right now, there is NO hum in anything! There are no covers installed at all and no tube shields in place and the 8000 has no hum discernible, at least, in my bench speakers. Gratifying, and we'll see how this holds up when I sound test it with the Klipsch speakers.
-The headphone audio is outstanding on FM and AUX sources. Classical music can be a real challenge because of the dynamic range and nature of the instruments (individual piano solo's will reveal flaws in audio reproduction). Bass is good a smooth in the phones on non-classical material through AUX.
-The Bimar EL506's are doing the job--these tubes are sounding really good and no untoward behavior so far (crackles, flashes, or glowing)--just good audio.
-Temperatures on the caps, transformers, and tubes all seem good after a couple hours of play.
So, the plan going forward (not necessarily in order):
1. Clean all switches, pots, and jacks. I have already done the RCA input jacks and the speaker terminals.
2. Replace couplers--at least the main couplers to the output tubes and maybe others. I have them all in hand, but the sound is pretty awesome right now, so I may choose to wait on the others until I listen through the main speakers (ANYONE HAVE EXPERIENCE WITH THE LIGHT BLUE COUPLER? THIS ONE HAS A BUNCH OF THEM BUT I HAVEN'T READ THEY ARE PARTICULARLY PRONE TO FAILURE).
3. Install 10 ohm cathode resistors and 100 ohm screen protection resistors. The bias level can be adjusted for each side, but not the balance between tubes. I will probably not mess with the bias circuit, but we'll see.
4. Re-install the dial cord and calibrate the dial.
5. Complete physical rehab, including cleaning up the front panel, maybe painting the tops of the transformers. I've cleaned the chassis and probably will leave it as it is now. It's clean and I did manage to get light rust off the worst parts (using a light and fast application of Naval Jelly followed by a fresh water rinse and polishing with Mother's polish. It looks good, but threatens the silk screened writing so I may just stop there. Clean up (or maybe paint) the tube shields.
6. Clean the wipers on the 3-gang tuning cap.
7. Re-install and re-solder the output tube shield (polished it up pretty nicely).
8. Fabricate at least a bottom cover from aluminum, probably, and install feet.
9. Further test the low-level inputs for response and hum (phono and tape head)
9. Depending on how the receiver acts when I hook it up to a real antenna, decide whether an alignment is needed. With 6 new tubes in the tuner section and new FM ratio detector diodes, this may be desirable. I have the tools to do it properly (including the MPX detector), but won't mess with it if the sensitivity, selectivity, and dial calibration seem OK. I'm thinking it will probably be necessary, though, at least for the receiver front end and IF strip. The multiplex decoder seems to be operating well and usually, if there is a problem with stereo, aligning the front end usually resolves it. We'll see.
10. Power and distortion test the amplifier.
I would appreciate any suggestions others may have about other work to do at this point, particularly from those who are familiar with Sherwood receivers or anyone with feedback or suggestions. I'm very pleased with what I'm hearing from this unit so far and look forward to getting it rehabbed and into the listening room for some longer-term listening and comparison with the other receivers I have.
Dave
Do you have a metal brake? That would make the bottom cover much easier to make. Also it makes me want to visit you since I need to make one for my S-5500 
Don't know the blue ones. Mine are all brown. They look like film, can't tell you anything else unfortunately.
Both my receiver and the integrated are very quiet. Seems almost amazing given the cramped quarters but apparently the designers knew their stuff. The tuner in the S-7000 really does a nice job too, and the S3MX MPX unit performs well. Not sure how similar the internal and external decoders are in terms of design, but I like what I hear from mine.
Don't know the blue ones. Mine are all brown. They look like film, can't tell you anything else unfortunately.
Both my receiver and the integrated are very quiet. Seems almost amazing given the cramped quarters but apparently the designers knew their stuff. The tuner in the S-7000 really does a nice job too, and the S3MX MPX unit performs well. Not sure how similar the internal and external decoders are in terms of design, but I like what I hear from mine.
I left some blue-green ones inside my 7700, partly because they're between the front panel almost buried...
You did an awesome job on this receiver and you're lucky to have good fm stereo (and being able to correct it yourself if it wasn't the case). Congrats!
Do you know the bias current on your new tubes?
You did an awesome job on this receiver and you're lucky to have good fm stereo (and being able to correct it yourself if it wasn't the case). Congrats!
Do you know the bias current on your new tubes?
Alas, no sheet metal brake. Will probably be improvising on that. I do know a guy who runs a sheet metal shop, but I hate to bother him for such a small project. I can probably get by with just a bottom plate and I'm not sure I'll have to do fancy bending on that. My son-in-law belongs to one of those "rent a workshop" spaces in Philly that may have some sheet metal gear.
As to the quiet nature, it no doubt a lot to do with design. I notice a fair number of twisted filament supply lines routed above the chassis on the S8000 IV. It looks a little funky, but must help with noise. There is a long ground bus below that the whole front end is grounded to.
The couplers for the output tubes are brown drops (0.068 uf at 600 V). The light blue ones don't have any brand on them I can see.
As I spend more time with this unit, I have become more appreciative of its' construction quality. This thing is really built solid!
There's a nice looking S7000 on the Bay that the guy has rigged up an outboard FM multiplex decoder.
Dave
As to the quiet nature, it no doubt a lot to do with design. I notice a fair number of twisted filament supply lines routed above the chassis on the S8000 IV. It looks a little funky, but must help with noise. There is a long ground bus below that the whole front end is grounded to.
The couplers for the output tubes are brown drops (0.068 uf at 600 V). The light blue ones don't have any brand on them I can see.
As I spend more time with this unit, I have become more appreciative of its' construction quality. This thing is really built solid!
There's a nice looking S7000 on the Bay that the guy has rigged up an outboard FM multiplex decoder.
Dave
Patrice,
Yes, I have heard these couplers described as "lavender," which I guess would be actually blue/green! No way to tell cathode current until I put the cathode resistors in (cathodes are all direct to ground), but I will measure and report it once I get them in. I expect to bias them like 7868/7591 counterparts, probably around 38 or 39 mA per tube.
Yes, I have heard these couplers described as "lavender," which I guess would be actually blue/green! No way to tell cathode current until I put the cathode resistors in (cathodes are all direct to ground), but I will measure and report it once I get them in. I expect to bias them like 7868/7591 counterparts, probably around 38 or 39 mA per tube.
Lavender... yes.
Biased my Sherwood at 32-33 ma like my 7591's amps (Heathkit AA100 and Scott LK-72). Was first at 55-60 ma, ouch!
Thinking that the four originals were bad (silly me, they had to be low to be tired!!), I replaced them with EH's: same high current even at minimum bias pots (lower I could go on the highest one was 46 ma)... I had to replace the 6.8k in the bias supply to 3.9k.
Now, I have four good Fisher branded 7868 but I already slacked the pins with new magnovals...
Biased my Sherwood at 32-33 ma like my 7591's amps (Heathkit AA100 and Scott LK-72). Was first at 55-60 ma, ouch!
Thinking that the four originals were bad (silly me, they had to be low to be tired!!), I replaced them with EH's: same high current even at minimum bias pots (lower I could go on the highest one was 46 ma)... I had to replace the 6.8k in the bias supply to 3.9k.
Now, I have four good Fisher branded 7868 but I already slacked the pins with new magnovals...
Well spotted, Patrice! I missed it with the coil shield wrapped around it. Haven't seen one shielded like this before. It is the ratio detector electrolytic--30 uF on the schematic. This are often replaced. I checked the ESR on it and it checks good, so I'm thinking about it. Did you replace in your unit? If the ratio detector is acting right, I may not.
